Ruby Falls is a 145-foot high underground waterfall located within Lookout Mountain, near Rock City and Chattanooga, Tennessee in the United States.The cave which houses Ruby Falls was formed with the formation of Lookout Mountain. About 200 to 240 million years ago (in the Carboniferous period, at the end of the Paleozoic era) the eastern Tennessee area was covered with a shallow sea, the sediments of which eventually formed limestone rock. About 200 million years ago, this area was uplifted and subsequent erosion has created the current topography. The limestone in which the cave is formed is still relatively horizontal, just as it was deposited when it was below sea level.

The Lookout Mountain Caverns, which includes Ruby Falls Cave, is a limestone cave. These caves occur when slightly acidic groundwater enters subterranean streams and eats away at the relatively soft limestone, causing narrow cracks to widen into passages and caves in a process called chemical weathering. The stream which makes up the Falls entered the cave sometime after its formation.The Falls are located at the end of the main passage of Ruby Falls Cave, in a large vertical shaft. The stream, 1120 feet underground, is fed both by rainwater and natural springs. It collects in a pool in the cave floor and then continues through the mountain until finally joining the Tennessee River at the base of Lookout Mountain.While Ruby Falls Cave combines with Lookout Mountain Cave to form the Lookout Mountain Caverns, the two caves were not actually connected by any passage. Ruby Falls Cave is the upper of the two and contains a variety of geological formations and curiosities which Lookout Mountain Cave does not have.

Electric lights were installed in the cave, making it one of the first commercial caves to be so outfitted. Motorists travelling on I-75 in the 70’s were subjected to dozens – maybe hundreds – of billboards along their route with the words “VISIT RUBY FALLS” beginning hundreds of miles north and south of the falls itself. Ruby Falls remains a staple of Chattanooga tourism, operating daily. Ruby Falls is owned by the Steiner family of Chattanooga, Tennessee.